Use pyupgrade to convert simple string formatting to use f-string
syntax. pyupgrade is intentionally timid and will not create an f-string
if it would make the expression longer or if the substitution parameters
are anything but simple names or dotted names.

Unnecessary since dropping Python 2 support. In Python 3, all classes
are new style classes.

Unnecessary since dropping Python 2.

Add a new testsuite option `--use-venv` to enable the use of `venv`
for creating a test virtual environment. The option is opt-in because
creating a `venv` environment does not work right when running under a
`virtualenv`; which is why `tox-venv` must be used in combination with
tox.

- cleanup virtualenv creation code
- ensure all testing virtual environments use a recent version
  of setuptools / wheel, making it easier to switch to custom
  versions of those, as well as reducing network accesses
- reduce size of testing virtual environment, slightly speeding
  up the testsuite

Changes were automated via https://github.com/asottile/pyupgrade

See #4921

Speedup virtualenv creation: create one (per session) relocatable
virtual environment, and then just make a copy of the resulting tree
when a new virtualenv is needed.

(cherry picked from commit b424e40d375851beea683f7dcdbe5134216a09b0)

Instead of:

    tests/conftest.py:146: in virtualenv
        pip_source_dir=pip_src,
    tests/lib/venv.py:45: in create
        obj._create(clear=clear)
    tests/lib/venv.py:68: in _create
        raise Exception(p.stderr)
    E   Exception: None

We get the more informative:

    tests/conftest.py:146: in virtualenv
        pip_source_dir=pip_src,
    tests/lib/venv.py:45: in create
        obj._create(clear=clear)
    tests/lib/venv.py:71: in _create
        output=p.stdout,
    E   CalledProcessError: Command '[Path(u'/private/var/folders/gw/w0clrs515zx9x_55zgtpv4mm0000gp/T/pytest-675/test_freeze_basic0/workspace/venv/bin/python'), 'setup.py', 'installxx']' returned non-zero exit status 1

For me, this speeds up `py.test tests/functional/test_freeze.py` from 62
seconds to 54 seconds (about a 13% speedup).

Do not hard code bin as the location of the executables, as this is not
correct on Windows.

Previously pip was always installed directly from the source tree
however this causes concurrency issues so it is now copied into
the temporary directory and installed from there.